While the present invention finds particular usefulness in the oil industry, it has particular application in other hostile pressure and temperature environments where size of the transducer and accuracy of the measurements are important.
In an oil well pressure transducer, it is common to size a sensor unit to go into a small diameter metal pressure housing for measurement of pressures up to 20,000 psi and even more and which can be subjected to downhole temperatures up to 400.degree. F. or more. The pressure housing must have a wall thickness sufficient to withstand the downhole pressures so that the O.D. of the internal pressure transducer or sensor is in the neighborhood of one inch in diameter.
A downhole oil well transducer or pressure gauge can be transported by a wireline, cable or pipe string to one or more levels of interest in a well bore where both temperature and pressure are sensed over a period of time. Typically, pressure measurements are repeatedly sampled and recorded over a period of time at a sampling rate determined by down hole electronics and may be stored in a downhole memory for subsequent replay or sent to the surface of analysis. Alternately, sometime pressure gauges are attached to production strings or other downhole equipment for measurements over extended periods of time or "permanently". In both uses of a pressure gauge, sudden downhole pressure changes can also typically accompanied by a temperature change in a relatively short period of time. It is also important for the accuracy of the pressure sensor to not change its calibration over a period of time in the borehole in response to pressure or temperature effects in the boreholes. Thus, there is a need for a pressure gauge for high pressure measurements which is also insensitive to sudden changes in temperature or effects of pressure. The present invention relates to high pressure transducers which can accurately measure high pressure changes under transient temperature conditions independently of the temperature changes.
Under the combined effect of high temperature and pressure conditions, the typical pressure sensor structure is subjected to high stress by the applied high pressure and subjected to high temperature both of which cause creep in the materials of the sensor structure. Creep in materials tends to be exponentially related to both temperature and stress levels. The effect of creep or permanent deformation in materials is to alter the calibration or measurement characteristics of a sensor and cause the sensor to obtain inaccurate measurements from its calibration standard over a period of time.
Heretofore, capacitance type transducers have been utilized for pressure measurements where a downhole oil well pressure varies an electrical capacitance as a function of pressure. The pressure is translated to a capacitance measurement by a pressure diaphragm moving parallel arranged capacitance plates toward and away from one another. An example of the kind of device is shown in U. S. Pat. No. 4,322,775.
I have also coupled a Bourdon tube to a capacitance type of sensor such as disclosed in U. S. Pat. No. 4,873,870 in which the pressure in the bourdon tube generates a directional linear force to displace quartz supported parallel arranged capacitor elements toward and away from one another. While this device is satisfactory for a number of applications, it is a difficult unit to manufacture.
One of the major concerns in utilizing downhole pressure sensors is that a downhole transducer malfunction can lose data or require re-running the downhole testing program with a different transducer. This can involve considerable expense and lost time. Another concern with the type of transducer disclosed herein is temperature sensitivity of different materials which can cause a drift of the measured pressure values over a period of time.
In the present invention, I have developed a capacitor device which has a unique relationship of capacitors that can be constructed from metals to respond to low force inputs and be relatively insensitive to temperature changes and has a redundancy feature to avoid abortion of a pressure testing program because of a malfunction. The capacitor device is particularly adaptable to measurement of high pressure with a high degree of accuracy and repeatability over a period of time.
As an example of probabilities of malfunctions, if one hundred single pressure sensors have a 5% failure rate then one unit out of every 20 units would fail in use. On the other hand if you have one hundred tandem connected pressure sensors with the same failure rate per unit, the probability that both sensors of a tandem unit will simultaneously fail is one out of 400 units. Thus, redundancy by use of tandem transducers can build substantial reliability into a downhole pressure sensor. Redundancy, however, also creates temperature problems since a transducer is matched to a base support with respect temperature expansion characteristics and separately mounted transducers can have different temperature expansion characteristics. The present invention provides for both redundancy and temperature compensation.